Soar Technology, Inc. proposes to develop a human-error modeling tool that can be used by designers and engineers early in the design process to identify likely causes of human error and engineer them out of the system. This tool will combine the strengths of an accurate high-fidelity model of human performance with a simple behavior description language for hierarchical task decomposition, and a usable developer interface. This tool will facilitate rapid modeling and analysis of interface candidates allowing a broader exploration of the design space and a deeper understanding of the design tradeoffs. This tool will result in lower development and operational costs for resulting designs, and safer, more effective, and more error-tolerant systems. Although some tools exist for analyzing designs for usability, and techniques exist for identifying human error, no tool or associated technique exists that is easy enough to be used by normal engineers on a regular basis. If successful, this work will represent a significant innovation in the areas of human-system interaction and error-tolerant design.